The present invention relates to a circuit device for driving an inductive load on the basis of a control signal.
There are known various conventional drive circuit devices for driving a solenoid (inductive load) of an electromagnetic valve. One of the known devices is disclosed in FIGS. 4(A) and 4(B) of Japanese Utility Model Application Laid-Open No. 61-108577. The drive circuit device disclosed in the Japanese utility model comprises its basic component including a current supply circuit having a power source and a drive transistor. The solenoid that is to be a subject of control is incorporated in the current supply circuit in series relation to the drive transistor.
In the above drive circuit device, current flowing through the solenoid is controlled without being subject to an influence of fluctuation in the power source voltage. Specifically, a detecting resistor is incorporated in the current supply circuit in series relation to the solenoid and the drive transistor. A voltage drop occurs at the detecting resistor in proportion to the current flowing through the solenoid. The dropped voltage serves as detecting voltage. Reference voltage is applied to a noninverting input terminal of an operational amplifier, while the detecting voltage from the detecting resistor is applied to an inverting input terminal of the operational amplifier. The operational amplifier controls base current of the drive transistor in such a manner that the voltage at the detecting resistor is made substantially equal to the reference voltage, thereby controlling the current flowing through the solenoid to a predetermined value.
In the above drive circuit device, current supply to the solenoid and stoppage of the current supply are controlled by a control signal. Specifically, a switching transistor has a collector-emitter circuit which is interposed between the base of the drive transistor and the ground side. The switching transistor is turned on and off in response to the control signal received by a base of the switching transistor. When the switching transistor is turned on, the drive transistor is turned off so that the current supply to the solenoid is stopped. On the other hand, when the switching transistor is turned off, the drive transistor is turned on so that the current supply to the solenoid is executed.
The above-described conventional drive circuit device has such a drawback that a remarkable overshoot occurs in the supply current subsequently to be described, when the current starts to be supplied to the solenoid.
Specifically, when the switching transistor is turned on and the drive transistor is turned off, the current does not flow through the solenoid, the drive transistor and the detecting resistor, so that the detecting voltage inputted to the operational amplifier from the detecting resistor is brought to zero. The operational amplifier outputs the maximum voltage (substantially equal to the power source voltage) in an attempt to raise the detecting voltage up to the reference voltage, so that the operational amplifier is saturated.
Accordingly, in a moment the switching transistor is turned off, high output voltage or large output current from the operational amplifier is supplied to the base of the drive transistor, so that the drive transistor is saturated temporarily. Thus, the supply current to the solenoid largely exceeds the predetermined value to be expected by control of the operational amplifier. Such a phenomenon that the supply current becomes excessive temporarily is called "overshoot".
The overshoot is uncontrollable and, therefore, hinders stable control of the solenoid. In case, for instance, where the above-mentioned drive circuit device is used for an electromagnetic valve which is opened and closed at high frequency, current supply to a solenoid of the electromagnetic valve and stoppage of the current supply are repeated at short intervals or cycles. Therefore, control of the electromagnetic valve is subject intensely to an influence of the overshoot occurring transiently. Since a peak value of the supply current at the overshoot fluctuates in response to the power source voltage, the control of the electromagnetic valve is subject to the influence of the fluctuation in the power source voltage. As a result, there may be a case where the timing of opening and closing of the electromagnetic valve gets out of order.
Further, after the overshoot, control by means of the operational amplifier causes the supply current to vary in a wave form and to converge into the aforesaid predetermined value at which the supply current is stabilized. During a period to a point of time the supply current is stabilized, the control of the solenoid is unstable.
Japanese Patent Application Laid-Open No. 61-144476 discloses such an arrangement that a Zener diode is interposed between the output terminal and the inverting input terminal of the operational amplifier, to limit the output voltage from the operational amplifier during stoppage of the current supply to the solenoid, thereby attempting to restrain the overshoot at the start-up of the current supply to the solenoid. Also in this case, however, a difference between the reference voltage at the noninverting input terminal of the operational amplifier and the voltage inputted to the inverting input terminal of the operational amplifier is not eliminated during stoppage of the current supply to the solenoid, so that the operational amplifier is saturated. Accordingly, the operational amplifier is not in a normal operating state in a moment the switching transistor is turned off and the drive transistor is turned on. Thus, it takes a time until the operational amplifier is returned to the normal operating state, so that the overshoot occurring during the period cannot sufficiently be restrained.